TOKYO -- With a surfeit of the old and a shortage of the young, Japan is on course for a population collapse unlike any in human history.

What ails this prosperous nation could be treated with babies and immigrants. Yet many young women here do not want children, and the Japanese will not tolerate a lot of immigrants. So government and industry are marching into the depopulated future with the help of robots -- some with wheels, some with legs, some that you can wear like an overcoat with muscles.

A small army of these machines, which has attracted huge and appreciative crowds, is on display this winter at the Great Robot Exhibition in Tokyo's National Museum of Nature and Science.

The Japanese are delighted by robots that look human. Honda's ASIMO can dance and serve tea. Toyota has a humanoid robot that plays "Pomp and Circumstance" on the violin -- rather robotically.

But engineers say it's the "service robots," which can't dance a lick and don't look remotely human, that can bail out Japan, which has the world's largest proportion of residents over 65 and smallest proportion of children under 15. One such gizmo, on display at the show, can spoon-feed the elderly. Others are being designed to hoist them onto a toilet and phone a nurse when they won't take their pills.

Toyota, the world's largest car company, announced last month that service robots would soon become one of its core businesses. The government heavily subsidizes development of these machines. Other cheerleaders for robots include universities and much of the news media.

Not everyone, though, is cheering. There are critics who describe the robot cure for an aging society as little more than high-tech quackery. They say that robots are a politically expedient palliative that allows politicians and corporate leaders to avoid wrenchingly difficult social issues, such as Japan's deep-seated aversion to immigration, its chronic shortage of affordable day care and Japanese women's increasing rejection of motherhood.

"Robots can be useful, but they cannot come close to overcoming the problem of population decline," said Hidenori Sakanaka, former head of the Tokyo Immigration Bureau and now director of the Japan Immigration Policy Institute, a research group in Tokyo.

"The government would do much better spending its money to recruit, educate and nurture immigrants," he said.

The scale of the coming demographic disaster, assuming present trends continue, is without precedent, according to Sakanaka and many other analysts.

Population shrinkage began here three years ago and is gathering pace. Within 50 years, the population, now 127 million, will fall by a third, the government projects. Within a century, two-thirds of the population will be gone. That would leave Japan, now the world's second-largest economy, with about 42 million people.

The workforce would shrink even faster, thanks to the dearth of children under 15, whose numbers have been falling for 26 consecutive years and now reflect a record-low 13.6 percent of the population.

Within 20 years, the workforce will fall by 10 percent, according to Goldman Sachs, the investment firm. It estimates that within 30 years, Japan will have just two workers for each retiree; within 50 years, two retirees for every three workers. Pension and health care systems will be at risk of collapse.

Robots can help make all this more affordable and less disruptive, said Masakatsu G. Fujie, a professor of mechanical engineering at Waseda University in Tokyo.

In a recent lecture to foreign journalists, he said service robots could help reduce government spending on health care, take over many dreary service jobs and prop up Japan's "societal vitality."

Still, if Japan is to have any chance of holding on to its status as a major economic power, it needs human beings by the millions, and it needs to start importing them soon, according to Sakanaka. He argues that Japan has no rational alternative but to open its doors to at least 10 million new immigrants over the next five decades.

This is a tall order. Among highly developed countries, Japan has always ranked near the bottom in the percentage of foreign-born residents. In the United States, about 12 percent are foreign-born; in Japan, just 1.6 percent.

Highly restrictive and aggressively enforced immigration laws have broad support from the Japanese public, which blames immigrants for crime, impolite behavior and untidiness. Sakanaka's immigration proposal, at least for the time being, has no serious backing among major political leaders.

But the country ranks first in robot use. Forty percent of the world's robots are at work here, mostly in industrial jobs.

The government prefers spending money on robot development rather than on immigrants, Sakanaka said, because robots do not have a political downside. "Politicians avoid the immigration issue because it doesn't lead to a vote," he said. "They should be thinking about Japan's future, but they are not."

Kathy Matsui, Japan strategist for Goldman Sachs, says robot promotion is a crowd-pleasing way for government and business to dance away from the core causes of Japan's low birthrate.

"Robots are simply not going to be able to do anything to deal with the problems of work and family," Matsui said. "Robots cannot raise kids."

And for all their potential, tending to an aging society with robots will not be easy. Designers say the machines -- mostly still in development and years away from entering the market -- must work safely, be affordable and make a profit for manufacturers. Industrial robots overcame many of these hurdles in the 1980s, and Japan's Ministry of Economy, Trade and Industry expects it to happen again with robots in the home.

"The ignition may be the dramatic decrease in the labor force," said Hideto Akiba, director of the ministry's industrial machine division.

A principal reason for the low birthrate in Japan is the increasing refusal of young women to marry. Government figures show that the percentage of women 25 to 29 who stay single has more than doubled since 1980, to 54 percent from 24 percent.

If Japanese women do marry and have children, they drop out of the workforce at far higher rates than women in other wealthy countries. The primary reason is because they cannot find affordable day care, according to Matsui and many others.

Matsui said affordable child care and relaxed immigration rules that allowed working mothers to hire foreign-born nannies would almost certainly keep more women in the workforce -- and could help raise the birthrate.

Asked why government and industry here are so taken with robots, Matsui said: "They are a nice excuse not to address the issue of immigration. They do not cause crime. They are not foreign people. And the Japanese are good at making robots."

At Toyota, robot-builders say it is not their job to answer big-picture questions.

They focus, instead, on how to make machines that help elderly people live comfortably and are safe, affordable and profitable.

In the next 10 to 20 years, Toyota contends, the most useful of these robots will be smart, highly mobile, wheelchair-like devices that bear little resemblance to robots in the movies.

"We are not focused on making robots that look like people," said Masashi Yamashta, general manager of Toyota's Partner Robot development division. "We aim to take the elderly outside with these machines."

The two-wheel "mobility robot" that Toyota introduced in Tokyo last month can carry a person over uneven ground or can act as a porter, following its owner with groceries or some other load. If the machines work well and are affordable, it is "realistic" that a partner robot will someday be in every home in Japan, Yamashta said.

"Are you going to let strangers into your home?" he asked. "Or do you have robots?"